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Some aspects of phase transformations in near-equiatomic niobium-ruthenium alloys
Some Aspects of Phase Transformations in Near-Eqniatomic Niobium-Ruthenium Alloys** B. K. DAS, M. A. SCHMERLING AND D. S. LIEBERMAN Department of Metalluroy and Minin9 Engineering and Materials Research Laboratory, University of lllinois, Urbana, Ill. 61801 (U.S.A.) (Received March 24, 1970)
S UMMA R Y Reversible phase transformations in three nearequiatomic Nb-Ru alloys have been studied usin9 electrical resistivity measurements, hot sta9 e optical metallography, X-ray diffraction and maonetie susceptibility measurements. A two-stage mechanism is
proposed for the transformations observed in these alloys; on coolin9 the high temperature cubic (~) phase transforms to a face-centered tetragonal (~') phase and the latter transforms to a face-centered orthorhombic (~") phase on further cool&9.
RI~SUMI~ Les transformations de phase r~versibles ont ~t~ ~tudi~es dans trois alliages Nb-Ru, de compositions voisines de la composition dquiatomique, gl l'aide des m~thodes suivantes : rdsistivitk ~lectrique, mOtallooraphie g~platine chauffante, diffraction des rayons X et mesures de susceptibilitO magnktique. Les transformations observ~es dans ces alliaoes sont
expliqu~es d l'aide d'un m~canisme en deux temps: au d~but du refroidissement la phase cubique de haute tempOrature ([3) se transforme en une phase quadratique h faces centrkes ([3'), puis celle-ci se transforme au cours du refroidissement ultkrieur en une phase orthorhombique ~ faces centr~es (fl").
Z USA M M E N F A SS UNG Reversible Phasenumwandlunoen in drei Nb-RuLegierungen mit Nb- und Ru-Konzentrationen nahe bei 50 A tomprozent wurden m it Hilfe yon elektrischen Widerstandsmessungen, yon optischen Messunoen in einer Heizpatrone, in R6ntoenbeuounosexperimenten und dureh Messun9 der magnetischen Suszeptibilitiit untersueht. Fiir die in diesen Legierungen beobachtete
Umwandlun9 wird ein Zweistufen-Mechanismus vor9esehlagen: Bei der Abkiihlun9 wandelt sich die kubische Hochtemperaturphase ([3) in eine fliichenzentrierte tetragonale Phase (~') urn; diese wiederum erleidet bei weiterem Abkiihlen eine Umwandlun9 in eine fliichenzentrierte orthorhombische Phase
INTRODUCTION
Nb-Ru alloys which have been reported to have many of the characteristics of the above-mentioned transformations as summarized here. Greenfield and Beck 3 reported that Nb-Ru alloys between 48 and 49 at. ~ Ru when quenched
The study of displacive or martensitic transformation has been stimulated by practical considerations as well as theoretical interests 1. Lieberman t,2 recently characterized martensitic or martensite-like transformations and classified the transformations and their products according to the range of atomic motion involved. The present work was undertaken to study transformations in near-equiatomic
(v).
* Based on a thesis submitted by B. K. Das to the University of Illinois, Urbana in partial fulfillment of the requirements for the degree of Master of Science in Metallurgical Engineering. t This work was supported in part by the U.S. Atomic Energy Commission under Contract AT(ll-1)-l198.
Materials Science and Engineering American Society for Metals, Metals Park, Ohio, and Elsevier Sequoia S.A., Lausanne - Printed in the Netherlands
PHASE TRANSFORMATIONS IN NEAR-EQUIATOMIC
from 1200° C to room temperature have a bodycentered tetragonal structure while an alloy containing 32 at. ~ Ru has a b.c.c, structure. Dwight 4 later reported parallel markings on a 47.6 at. ~ Ru specimen quenched from 1200° C to room temperature. Raub and Fritzsche 5 confirmed the presence of a body-centered tetragonal phase between 41 and 46.5 at. ~o Ru and further reported that alloys having composition between 47 and 58 at. ~ Ru have a face-centered orthorhombic structure. This paper describes studies of the transformations above room temperature in these alloys using resistometric, metallographic, X-ray diffraction and magnetic susceptibility techniques.
s
Nb-Ru ALLOYS
249
~ .df°'~'R~, c o 4 [ ~ '0
(4S'Sat%eu) I 0 heating
~,~
~5.8 5.6 0
200
400 Temp°C
600
800
Fig. 1. Electrical resistance of alloy 1 (45.8 at. % Re) vs. temperature. Note the absence of any hysteresis within experimental errors.
EXPERIMENTAL TECHNIQUES
Specimens for electrical resistance measurement were prepared by electron beam melting die compressed powder compacts (2 in. long and 1/8 in. square) of Nb (99.95 wt. ~ purity) and Ru (99.98 wt. ~ purity). Three alloys of compositions 45.8, 51.1 and 55.8 at. ~ Ru were prepared. The electrical resistance was determined by passing a known current through the specimen and measuring the voltage drop across it. The specimen could be heated up to 1300° C under a vacuum of better than 5 x 10- 6 torr in a high frequency induction furnace. Hot stage metallography was done using a Lietz heating stage. The room temperature lattice parameters were determined by using a 5.73 cm diameter Phillips powder camera. The apparatus used for magnetic susceptibility measurement has been described elsewhere6.
I
i
6,0 E a~z o 5,8
% x
5.6
g 5.4 co
~, 5.2 n2
/ /
] L
0 heating A cooling
5,0 ~ _ _ _
I
400
i
i
600 800 Temp°C
1000
Fig. 2. Electrical resistance of alloy 2 (51.1 at. % Ru) vs. temperature. Note the change in the slope of the curve at about 790° C.
3.20
I
I
I
1 - -
RESULTS
In all three alloys studied the departure of the electrical resistance v e r s u s temperature curves from linearity on both heating and cooling (Figs. 1-3) and concomitant changes in microstructure observed metallographically were associated with phase transformation. Although the transformation regions extended over several hundred degrees the resistance v e r s u s temperature curves exhibited no hysteresis within + 10degC in any of the three alloys. Different heating rates (from 50 deg C/h to 300 deg C/h) produced little effect on the details of the resistance changes. On cooling alloy 1 (45.8 at. ~ Ru) started to transform at 525° C and the end of the
xz 3.15 o
% × 3.10
3.05 ?' or"
rtO /'" A 3,00 ~)
( 55.8 at °/oRUH 0 heating J A cooling
O '
0
800
~
1000 Temp°C
,
,
1200
1400
Fig. 3. Electrical resistance of alloy 3 (55.8 at. % Ru) v s . temperature. The upper limit of temperature that could be obtained in the apparatus used was 1300 ° C.
M a t e r . Sci. Eng., 6 (1970) 248-254
250
B. K. DAS, M. A. SCHMERLING, D. S. LIEBERMAN
transformation was 150° C. These two characteristic temperatures will henceforth be referred to as Ms and M e respectively in keeping with the common notation for martensitic phase transformation. The slope of the resistance v e r s u s temperature curve for alloy 1 (Fig. 1) remained essentially constant during the transformation. In case of alloy 2 (51.1 at. ~ Ru) with an M s of 900° C and M e of 700° C a definite change in slope of the resistance v e r s u s temperature curve (Fig. 2) at about 790° C was observed on both heating and cooling; hysteresis seemed to be absent in the more complex behavior in alloy 2 also. Mf for alloy 3 (55.8 at. Ru) was 900° C (Fig. 3); completion of the transformation on heating could not be determined for this alloy in the apparatus used.
From the transformation temperature it is clear that polishing these alloys in the high temperature parent phase would present formidable difficulties. Hence the "inverse" upheavals and markings on a surface polished in the room temperature product phase following initial transformation on cooling after growth were observed in the hot stage metallograph as the specimen was heated up through the first reverse transformation ; the marked surface was considered the 'reference' state. Parallel markings appeared on the surface (Fig. 4a) of alloy 1 (45.8 at. Ru), which had been polished at room temperature, indicating the start of the reverse reaction from the product to the parent phase. When heated to 1600° C and cooled a second set of markings at an angle to those previously mentioned appeared at
Fig. 4. (a--c) Photomicrographs of the surface of alloy 1 (45.8 at. ~o Ru) polished at room temperature just after growth, heated to 230 ° C in (a), then heated to 1600 ° C and cooled to 490 ° C in (b), and then cooled to room temperature in (c). x 500.
Mater. Sci. Eng., 6 (1970) 248-254
PHASE TRANSFORMATIONS IN NEAR-EQUIATOMIC
Nb-Ru
ALLOYS
251
Fig. 5. (a~l) Photomicrographs of the surface of alloy 2 (51.1 at. ~ R u ) polished at room temperature just after growth, heated to 750 ° C in (a), then heated to 830° C in (b), further heated to 1000° C in (c), (d) shows the surface after it was cooled from the conditions of(c) to room temperature, and subsequently heated to 1600° C and cooled to room temperature, x 100.
490 ° C (Fig. 4b) indicating that a parent to product reaction had started on another set of planes, presumably crystallographically equivalent to the first. On cycling between 550° C and 100° C the second set disappeared on heating and reappeared on cooling confirming the transformation temperatures obtained from electrical resistance measurements. On heating alloy 2 (51.1 at. ~ Ru), polished at room temperature, markings arranged in chevron-like bands appeared at 700 ° C; the surface at 750~ C is shown in Fig. 5a. On further heating the broad bands enclosing the finer markings began to tilt relative to each other at 800° C ; Figs. 5b and 5c show the surface at 830 ° C and 1000° C respectively. During cooling to room temperature the broad bands tilted back resulting in a surface which was almost flat, but the finer markings inside them re-
mained. However, on heating to 1600° C and cooling to room temperature a second set of alternate dark and bright bands (Fig. 5d) appeared. This indicates that although there was no hysteresis exhibited in this alloy, it had to be heated well above its M s into the parent phase to remove the "memory" of the substructure in the product. Such effect has been observed in AuCd v. During the cycling between 1000° C and 700° C the second set of bands disappeared on heating and reappeared on cooling. In alloy 3 (55.8 at. ~ Ru) parallel markings appeared at 900° C on the surface polished at room temperature. When the alloy was heated to 1600° C and cooled to 800° C, parallel markings at an angle to the original ones appeared (Fig. 6a). During the cycling between 1300° C and 800° C the markings which appeared on cooling from 1600° C to 800 ° C Mater. Sei. Eng., 6 (1970) 248 254
252
B . K . DAS, M. A. S C H M E R L I N G , D. S. LIEBERMAN
Fig. 6. (a-c) P h o t o m i c r o g r a p h s of the surface of alloy 3 (55.8 at. % Ru) polished at r o o m t e m p e r a t u r e just after growth, h e a t e d to 1600 ° C a n d c o o l e d to 800 ° C in (a), then h e a t e d to 1300 ° C in (b), a n d t h e n c o o l e d to 800 ° C in (c). x 400.
disappeared on heating (Fig. 6b) and reappeared on cooling (Fig. 6c). The "reference" state markings, as described above, always appeared upon heating to the high temperature phase. The room temperature crystal structure and lattice parameters for the three alloys as determined on powders in this study are given in Table 1.
To ascertain whether there was any effect of particle size on the transformation magnetic susceptibility of both bulk and powder specimens was measured. Alloy 2 (51.1 at. % Ru) was chosen for the magnetic study; the force on the alloy, which (after quartz capsule correction 6,s) is directly proportional to the magnetic susceptibility, is shown as a function of temperature for bulk and powder specimens in Figs. 7 and 8 respectively. The high temperature parent phase was observed to be paramagnetic and its magnetic susceptibility showed no temperature dependence ; the low temperature phase also exhibited paramagnetism, but its magnetic susceptibility decreased with decreasing temperature. The transformations occurred over the same temperature region as indicated by electrical resistance measurements for both bulk and powder specimens. A change in the slope of the curve during the transformation
T A B L E 1 : CRYSTAL STRUCTURE AND LATTICE PARAMETER OF N b - R u ALLOYS
Alloy #
Composition (at. % Ru)
Crystal structure
Lattice parameter
c/a
c/b
1
45.8
Face-centered tetragonal
a = 4.388 _ 0.005 c = 3.311 + 0.005
0.755
2
51.1
Face-centered orthorhombic
a =4.373_0.005 b = 4.228 + 0.005 c = 3.401 + 0.005
0.788
0.804
3
55.8
Face-centered orthrohombic
a=4.295 +0.005 b=4.192+0.005 c = 3.439 +0.005
0.801
0.820
Mater. Sci. Eng., 6 (1970) 248-254
PHASE TRANSFORMATIONS IN NEAR-EQUIATOMIC
0.24
[NbLRu Alloy 22 I
Bulk
LA cooling 0.20
]~ - - - ~
~
t
. . . . .
400
fl
I
]
600 800 Temp °C
I
tO0O
-
Fig. 7. Variation of magnetic susceptibility of very large grained specimens of alloy 2 (51.1 at. o/ /o Ru) with temperature. The susceptibility is directly proportional to the force (after quartz correction6'S). The susceptibility at 1000~ C was 1.03 × 10 6 emu/g. Compare with Figs. 2, 5 and 8.
O.24 E 0.22 X 0.20 v
Nb-Au Atloy2 (51.1at %Ru Powder ] O heating [A cooling
o.~8 o w oA6 J
400
J
600 800 Temp °C
i000
Fig. 8. Change in magnetic susceptibility of powders of alloy 2 (51.1 at. ~ Ru) with temperature. Compare with Figs. 2, 5 and 7.
occurred essentially at the same temperature as the break in resistance versus temperature curve of Fig. 2; the break in the powder curve was not as clearly defined. The discrepancies between the bulk and the powder curves can be explained by the anisotropic nature of the bulk specimens, which were very large grained.
DISCUSSION AND CONCLUSIONS
It will be noticed that the transformation temperatures decrease rapidly with decreasing Ru content, going from over 1300 ° C to 525 ° C as the composition changes from 55.8 to 45.8 at. ~ Ru. Thus if there are transformations in alloys of lower Ru content, they would occur below room temperature before the 30 at. ~ Ru alloy, for which Greenfield and Beck 2 reported a b.c.c, or possibly CsC1 structure (no super lattice lines were observed since the
Nb-Ru ALLOYS
253
difference in scattering powers of Nb and Ru is small) in specimens quenched from 1200° C. Hence it will be assumed that the high temperature parent phase for the transformation reported here is CsC1 for the 50-50 composition and highly ordered for offstoichiometry alloys. This assumption is consistent with the results of the recent study of similar transformations in near-equiatomic Ta-Ru alloys 8, where definite super lattice lines were observed ; such lines should be more easily observed since the atomic numbers of Ta(73) and Ru(44) are reasonably different while those of Nb(41) and Ru(44) are very close. Raub and Fritzsche s reported the low temperature phase to be body-centered tetragonal in alloys having compositions between 4l and 46.5 at. ~, Ru and face-centered orthorhombic for alloys having compositions between 47 and 58 at. ~oRu. However, to better understand the relationship among the product phases and how they are related to the parent phase, it is much more instructive to choose a face-centered tetragonal cell instead of the bodycentered tetrayonal cell in the former composition range. Such a cell is delineated within four bodycentered tetragonal cells in Fig. 9b, where ae is the a parameter of the face-centered tetragonal cell, equal t o ~ a b , the a parameter of the body-centered tetragonal cell. Thus, it can be seen that the cell in Fig. 9b and the lower symmetry face-ccntcred orthorhombic cell in Fig. 9c are related to each other and to an arbitrary face-centered tetragonal unit cell with c/a = 1/,,/2 shown dashed in the parent cubic phase in Fig. 9a. This rather unified approach to all transformations in the composition range examined includes the possibility that the f.c.o, cell in Fig. 9c could result from a sequence of transformations from the parent cubic phase to a f.c.t, phase to a f.c.o, phase as the temperature is decreased. The high temperature CsC1 phase, the face-centered tetragonal phase and the face-centered orthorhombic phase will henceforth be referred to as/3, If and/3" respectively. Thus, the behavior of alloy 2 (51.1 at. °/o Ru) can be interpreted as follows. From Figs. 1 and 2 it can be seen that the slope of the curve for alloy 1 is essentially constant and the same as the slope of the curve for alloy 2 between 800° C and 900 ° C. Since the low temperature phase and the high temperature phases for alloy 1 are/3' and/3 respectively, it thus appears that in alloy 2 the high temperature/3 phase transforms to fl' on cooling and this/3' phase transforms to/3" on further cooling. The start of the ff to Mater. Sci. Eng., 6 (1970) 248-254
254
B . K . DAS, M. A. SCHMERLING, D. S. LIEBERMAN
~Ru
ATOMS @ N b A T O M S
°,~5.0 = -o
,.
Nb-Ru Alloys IO,A,rn present investigation 5.6 Lo,A,m data of Raub & Fritzsche5
-_-_~ \
5.4
~ 4.6 5.2 ~ t3_
5.o ~
(a)
(~)
f.c.t.
Fig. 10. Variation of room temperature lattice parameters of near-equiatomic N b - R u alloys with composition.
CsCI
(c)
f.c.o.
Fig. 9. (a) Untransformed high temperature parent CsC1 type structure in which a f.c.t, cell is delineated with a = x/2a0, c/a = l/x/2, (b) face-centered tetragonal cell obtained on account of transformation distortions; the (equally good) body-centred tetragonal cell is also shown, (c) face-centered orthorhombic cell produced because of a distortion which is anisotropic in the plane perpendicular to the 'c' axis.
this curve shows a direct transformation from fl to fl". The observations of Raub and Fritzsche s corroborate the proposed two-step transformation process. Their data are plotted together with the results of this investigation in Fig. 10 after converting their b.c.t, cells to f.c.t, cells by multiplying their a values by \/2. It can be seen from this Figure that the f.c.o. cell differs very little form the f.c.t, cell and it could be derived from the cubic phase by the suggested two-step process with an intermediate f.c.t, phase. Furthermore, even room temperature c/a and c/b ratios are not very different from 1/x/2=0.707, consistent with the model proposed; c/a and c/b ratios may be very close to 0.707 at transformation temperatures since absence of hysteresis indicates very small strains.
ACKNOWLEDGEMENTS
fl" transformation occurs before completion of fl to fl' transformation; this accounts for the change in slopes of the resistance and magnetic susceptibility versus temperature curves during transformation. This model is also consistent with the hot stage metallographic observations on alloy 2. The appearance of the fine markings (Figs. 5a) arranged in chevron-like bands on the surface polished at room temperature and heated to 750° C is consistent with a reverse reaction if' to fl' while tilting of the broad bands above 800 ° C (Fig. 5b) fits with the fl' to fl reaction. In case of alloy 1 the fl to fl' reaction is complete above room temperature; there may indeed be a ff to fl" transformation below room temperature. In case of alloy 3 it cannot be stated at this time whether a fl to fl' transformation precedes the transformation indicated by Fig. 3, which would then be interpreted as fl' to fl" on cooling, or whether
The authors wish to thank Mr. R. Hoffman and Mr. J. Peters for their aid in crystal preparation and susceptibility measurements respectively.
REFERENCES 1 D. S. LIEBERMAN, The mechanism of phase transformations in crystalline solids, Institute of Metals Monograph No. 33, 167, London, 1969. 2 D. S. LIEBERMAN, Am. Soc. Metals Seminar on Phase Transformations, Detroit, 12-13 October, 1968, in press. 3 P. GREENFIELD AND ]~. A. BECK, Trans. AIME, 206 (1956) 265. 4 A. E. DWIGHT, Trans. AIME, 215 (1959) 283. 5 E. RAUB A~qDW. FRITZSCHE, Z. Metallk., 54 (1963) 317. 6 J. A. GARDNER AND C. P. FLV~qIq, Phil. Mag., 15 (1967) 1233. 7 S.G. FISHMAN, R. KARZ AND D. S. LmBERMAN,to be published. 8 M. A. SCI-IMERLTNG,B. K. DAS AND D. S. LmBERMAN, Met. Trans., in press.
Mater. Sci. Eno., 6 (1970} 248-254
S UMMA R Y Reversible phase transformations in three nearequiatomic Nb-Ru alloys have been studied usin9 electrical resistivity measurements, hot sta9 e optical metallography, X-ray diffraction and maonetie susceptibility measurements. A two-stage mechanism is
proposed for the transformations observed in these alloys; on coolin9 the high temperature cubic (~) phase transforms to a face-centered tetragonal (~') phase and the latter transforms to a face-centered orthorhombic (~") phase on further cool&9.
RI~SUMI~ Les transformations de phase r~versibles ont ~t~ ~tudi~es dans trois alliages Nb-Ru, de compositions voisines de la composition dquiatomique, gl l'aide des m~thodes suivantes : rdsistivitk ~lectrique, mOtallooraphie g~platine chauffante, diffraction des rayons X et mesures de susceptibilitO magnktique. Les transformations observ~es dans ces alliaoes sont
expliqu~es d l'aide d'un m~canisme en deux temps: au d~but du refroidissement la phase cubique de haute tempOrature ([3) se transforme en une phase quadratique h faces centrkes ([3'), puis celle-ci se transforme au cours du refroidissement ultkrieur en une phase orthorhombique ~ faces centr~es (fl").
Z USA M M E N F A SS UNG Reversible Phasenumwandlunoen in drei Nb-RuLegierungen mit Nb- und Ru-Konzentrationen nahe bei 50 A tomprozent wurden m it Hilfe yon elektrischen Widerstandsmessungen, yon optischen Messunoen in einer Heizpatrone, in R6ntoenbeuounosexperimenten und dureh Messun9 der magnetischen Suszeptibilitiit untersueht. Fiir die in diesen Legierungen beobachtete
Umwandlun9 wird ein Zweistufen-Mechanismus vor9esehlagen: Bei der Abkiihlun9 wandelt sich die kubische Hochtemperaturphase ([3) in eine fliichenzentrierte tetragonale Phase (~') urn; diese wiederum erleidet bei weiterem Abkiihlen eine Umwandlun9 in eine fliichenzentrierte orthorhombische Phase
INTRODUCTION
Nb-Ru alloys which have been reported to have many of the characteristics of the above-mentioned transformations as summarized here. Greenfield and Beck 3 reported that Nb-Ru alloys between 48 and 49 at. ~ Ru when quenched
The study of displacive or martensitic transformation has been stimulated by practical considerations as well as theoretical interests 1. Lieberman t,2 recently characterized martensitic or martensite-like transformations and classified the transformations and their products according to the range of atomic motion involved. The present work was undertaken to study transformations in near-equiatomic
(v).
* Based on a thesis submitted by B. K. Das to the University of Illinois, Urbana in partial fulfillment of the requirements for the degree of Master of Science in Metallurgical Engineering. t This work was supported in part by the U.S. Atomic Energy Commission under Contract AT(ll-1)-l198.
Materials Science and Engineering American Society for Metals, Metals Park, Ohio, and Elsevier Sequoia S.A., Lausanne - Printed in the Netherlands
PHASE TRANSFORMATIONS IN NEAR-EQUIATOMIC
from 1200° C to room temperature have a bodycentered tetragonal structure while an alloy containing 32 at. ~ Ru has a b.c.c, structure. Dwight 4 later reported parallel markings on a 47.6 at. ~ Ru specimen quenched from 1200° C to room temperature. Raub and Fritzsche 5 confirmed the presence of a body-centered tetragonal phase between 41 and 46.5 at. ~o Ru and further reported that alloys having composition between 47 and 58 at. ~ Ru have a face-centered orthorhombic structure. This paper describes studies of the transformations above room temperature in these alloys using resistometric, metallographic, X-ray diffraction and magnetic susceptibility techniques.
s
Nb-Ru ALLOYS
249
~ .df°'~'R~, c o 4 [ ~ '0
(4S'Sat%eu) I 0 heating
~,~
~5.8 5.6 0
200
400 Temp°C
600
800
Fig. 1. Electrical resistance of alloy 1 (45.8 at. % Re) vs. temperature. Note the absence of any hysteresis within experimental errors.
EXPERIMENTAL TECHNIQUES
Specimens for electrical resistance measurement were prepared by electron beam melting die compressed powder compacts (2 in. long and 1/8 in. square) of Nb (99.95 wt. ~ purity) and Ru (99.98 wt. ~ purity). Three alloys of compositions 45.8, 51.1 and 55.8 at. ~ Ru were prepared. The electrical resistance was determined by passing a known current through the specimen and measuring the voltage drop across it. The specimen could be heated up to 1300° C under a vacuum of better than 5 x 10- 6 torr in a high frequency induction furnace. Hot stage metallography was done using a Lietz heating stage. The room temperature lattice parameters were determined by using a 5.73 cm diameter Phillips powder camera. The apparatus used for magnetic susceptibility measurement has been described elsewhere6.
I
i
6,0 E a~z o 5,8
% x
5.6
g 5.4 co
~, 5.2 n2
/ /
] L
0 heating A cooling
5,0 ~ _ _ _
I
400
i
i
600 800 Temp°C
1000
Fig. 2. Electrical resistance of alloy 2 (51.1 at. % Ru) vs. temperature. Note the change in the slope of the curve at about 790° C.
3.20
I
I
I
1 - -
RESULTS
In all three alloys studied the departure of the electrical resistance v e r s u s temperature curves from linearity on both heating and cooling (Figs. 1-3) and concomitant changes in microstructure observed metallographically were associated with phase transformation. Although the transformation regions extended over several hundred degrees the resistance v e r s u s temperature curves exhibited no hysteresis within + 10degC in any of the three alloys. Different heating rates (from 50 deg C/h to 300 deg C/h) produced little effect on the details of the resistance changes. On cooling alloy 1 (45.8 at. ~ Ru) started to transform at 525° C and the end of the
xz 3.15 o
% × 3.10
3.05 ?' or"
rtO /'" A 3,00 ~)
( 55.8 at °/oRUH 0 heating J A cooling
O '
0
800
~
1000 Temp°C
,
,
1200
1400
Fig. 3. Electrical resistance of alloy 3 (55.8 at. % Ru) v s . temperature. The upper limit of temperature that could be obtained in the apparatus used was 1300 ° C.
M a t e r . Sci. Eng., 6 (1970) 248-254
250
B. K. DAS, M. A. SCHMERLING, D. S. LIEBERMAN
transformation was 150° C. These two characteristic temperatures will henceforth be referred to as Ms and M e respectively in keeping with the common notation for martensitic phase transformation. The slope of the resistance v e r s u s temperature curve for alloy 1 (Fig. 1) remained essentially constant during the transformation. In case of alloy 2 (51.1 at. ~ Ru) with an M s of 900° C and M e of 700° C a definite change in slope of the resistance v e r s u s temperature curve (Fig. 2) at about 790° C was observed on both heating and cooling; hysteresis seemed to be absent in the more complex behavior in alloy 2 also. Mf for alloy 3 (55.8 at. Ru) was 900° C (Fig. 3); completion of the transformation on heating could not be determined for this alloy in the apparatus used.
From the transformation temperature it is clear that polishing these alloys in the high temperature parent phase would present formidable difficulties. Hence the "inverse" upheavals and markings on a surface polished in the room temperature product phase following initial transformation on cooling after growth were observed in the hot stage metallograph as the specimen was heated up through the first reverse transformation ; the marked surface was considered the 'reference' state. Parallel markings appeared on the surface (Fig. 4a) of alloy 1 (45.8 at. Ru), which had been polished at room temperature, indicating the start of the reverse reaction from the product to the parent phase. When heated to 1600° C and cooled a second set of markings at an angle to those previously mentioned appeared at
Fig. 4. (a--c) Photomicrographs of the surface of alloy 1 (45.8 at. ~o Ru) polished at room temperature just after growth, heated to 230 ° C in (a), then heated to 1600 ° C and cooled to 490 ° C in (b), and then cooled to room temperature in (c). x 500.
Mater. Sci. Eng., 6 (1970) 248-254
PHASE TRANSFORMATIONS IN NEAR-EQUIATOMIC
Nb-Ru
ALLOYS
251
Fig. 5. (a~l) Photomicrographs of the surface of alloy 2 (51.1 at. ~ R u ) polished at room temperature just after growth, heated to 750 ° C in (a), then heated to 830° C in (b), further heated to 1000° C in (c), (d) shows the surface after it was cooled from the conditions of(c) to room temperature, and subsequently heated to 1600° C and cooled to room temperature, x 100.
490 ° C (Fig. 4b) indicating that a parent to product reaction had started on another set of planes, presumably crystallographically equivalent to the first. On cycling between 550° C and 100° C the second set disappeared on heating and reappeared on cooling confirming the transformation temperatures obtained from electrical resistance measurements. On heating alloy 2 (51.1 at. ~ Ru), polished at room temperature, markings arranged in chevron-like bands appeared at 700 ° C; the surface at 750~ C is shown in Fig. 5a. On further heating the broad bands enclosing the finer markings began to tilt relative to each other at 800° C ; Figs. 5b and 5c show the surface at 830 ° C and 1000° C respectively. During cooling to room temperature the broad bands tilted back resulting in a surface which was almost flat, but the finer markings inside them re-
mained. However, on heating to 1600° C and cooling to room temperature a second set of alternate dark and bright bands (Fig. 5d) appeared. This indicates that although there was no hysteresis exhibited in this alloy, it had to be heated well above its M s into the parent phase to remove the "memory" of the substructure in the product. Such effect has been observed in AuCd v. During the cycling between 1000° C and 700° C the second set of bands disappeared on heating and reappeared on cooling. In alloy 3 (55.8 at. ~ Ru) parallel markings appeared at 900° C on the surface polished at room temperature. When the alloy was heated to 1600° C and cooled to 800° C, parallel markings at an angle to the original ones appeared (Fig. 6a). During the cycling between 1300° C and 800° C the markings which appeared on cooling from 1600° C to 800 ° C Mater. Sei. Eng., 6 (1970) 248 254
252
B . K . DAS, M. A. S C H M E R L I N G , D. S. LIEBERMAN
Fig. 6. (a-c) P h o t o m i c r o g r a p h s of the surface of alloy 3 (55.8 at. % Ru) polished at r o o m t e m p e r a t u r e just after growth, h e a t e d to 1600 ° C a n d c o o l e d to 800 ° C in (a), then h e a t e d to 1300 ° C in (b), a n d t h e n c o o l e d to 800 ° C in (c). x 400.
disappeared on heating (Fig. 6b) and reappeared on cooling (Fig. 6c). The "reference" state markings, as described above, always appeared upon heating to the high temperature phase. The room temperature crystal structure and lattice parameters for the three alloys as determined on powders in this study are given in Table 1.
To ascertain whether there was any effect of particle size on the transformation magnetic susceptibility of both bulk and powder specimens was measured. Alloy 2 (51.1 at. % Ru) was chosen for the magnetic study; the force on the alloy, which (after quartz capsule correction 6,s) is directly proportional to the magnetic susceptibility, is shown as a function of temperature for bulk and powder specimens in Figs. 7 and 8 respectively. The high temperature parent phase was observed to be paramagnetic and its magnetic susceptibility showed no temperature dependence ; the low temperature phase also exhibited paramagnetism, but its magnetic susceptibility decreased with decreasing temperature. The transformations occurred over the same temperature region as indicated by electrical resistance measurements for both bulk and powder specimens. A change in the slope of the curve during the transformation
T A B L E 1 : CRYSTAL STRUCTURE AND LATTICE PARAMETER OF N b - R u ALLOYS
Alloy #
Composition (at. % Ru)
Crystal structure
Lattice parameter
c/a
c/b
1
45.8
Face-centered tetragonal
a = 4.388 _ 0.005 c = 3.311 + 0.005
0.755
2
51.1
Face-centered orthorhombic
a =4.373_0.005 b = 4.228 + 0.005 c = 3.401 + 0.005
0.788
0.804
3
55.8
Face-centered orthrohombic
a=4.295 +0.005 b=4.192+0.005 c = 3.439 +0.005
0.801
0.820
Mater. Sci. Eng., 6 (1970) 248-254
PHASE TRANSFORMATIONS IN NEAR-EQUIATOMIC
0.24
[NbLRu Alloy 22 I
Bulk
LA cooling 0.20
]~ - - - ~
~
t
. . . . .
400
fl
I
]
600 800 Temp °C
I
tO0O
-
Fig. 7. Variation of magnetic susceptibility of very large grained specimens of alloy 2 (51.1 at. o/ /o Ru) with temperature. The susceptibility is directly proportional to the force (after quartz correction6'S). The susceptibility at 1000~ C was 1.03 × 10 6 emu/g. Compare with Figs. 2, 5 and 8.
O.24 E 0.22 X 0.20 v
Nb-Au Atloy2 (51.1at %Ru Powder ] O heating [A cooling
o.~8 o w oA6 J
400
J
600 800 Temp °C
i000
Fig. 8. Change in magnetic susceptibility of powders of alloy 2 (51.1 at. ~ Ru) with temperature. Compare with Figs. 2, 5 and 7.
occurred essentially at the same temperature as the break in resistance versus temperature curve of Fig. 2; the break in the powder curve was not as clearly defined. The discrepancies between the bulk and the powder curves can be explained by the anisotropic nature of the bulk specimens, which were very large grained.
DISCUSSION AND CONCLUSIONS
It will be noticed that the transformation temperatures decrease rapidly with decreasing Ru content, going from over 1300 ° C to 525 ° C as the composition changes from 55.8 to 45.8 at. ~ Ru. Thus if there are transformations in alloys of lower Ru content, they would occur below room temperature before the 30 at. ~ Ru alloy, for which Greenfield and Beck 2 reported a b.c.c, or possibly CsC1 structure (no super lattice lines were observed since the
Nb-Ru ALLOYS
253
difference in scattering powers of Nb and Ru is small) in specimens quenched from 1200° C. Hence it will be assumed that the high temperature parent phase for the transformation reported here is CsC1 for the 50-50 composition and highly ordered for offstoichiometry alloys. This assumption is consistent with the results of the recent study of similar transformations in near-equiatomic Ta-Ru alloys 8, where definite super lattice lines were observed ; such lines should be more easily observed since the atomic numbers of Ta(73) and Ru(44) are reasonably different while those of Nb(41) and Ru(44) are very close. Raub and Fritzsche s reported the low temperature phase to be body-centered tetragonal in alloys having compositions between 4l and 46.5 at. ~, Ru and face-centered orthorhombic for alloys having compositions between 47 and 58 at. ~oRu. However, to better understand the relationship among the product phases and how they are related to the parent phase, it is much more instructive to choose a face-centered tetragonal cell instead of the bodycentered tetrayonal cell in the former composition range. Such a cell is delineated within four bodycentered tetragonal cells in Fig. 9b, where ae is the a parameter of the face-centered tetragonal cell, equal t o ~ a b , the a parameter of the body-centered tetragonal cell. Thus, it can be seen that the cell in Fig. 9b and the lower symmetry face-ccntcred orthorhombic cell in Fig. 9c are related to each other and to an arbitrary face-centered tetragonal unit cell with c/a = 1/,,/2 shown dashed in the parent cubic phase in Fig. 9a. This rather unified approach to all transformations in the composition range examined includes the possibility that the f.c.o, cell in Fig. 9c could result from a sequence of transformations from the parent cubic phase to a f.c.t, phase to a f.c.o, phase as the temperature is decreased. The high temperature CsC1 phase, the face-centered tetragonal phase and the face-centered orthorhombic phase will henceforth be referred to as/3, If and/3" respectively. Thus, the behavior of alloy 2 (51.1 at. °/o Ru) can be interpreted as follows. From Figs. 1 and 2 it can be seen that the slope of the curve for alloy 1 is essentially constant and the same as the slope of the curve for alloy 2 between 800° C and 900 ° C. Since the low temperature phase and the high temperature phases for alloy 1 are/3' and/3 respectively, it thus appears that in alloy 2 the high temperature/3 phase transforms to fl' on cooling and this/3' phase transforms to/3" on further cooling. The start of the ff to Mater. Sci. Eng., 6 (1970) 248-254
254
B . K . DAS, M. A. SCHMERLING, D. S. LIEBERMAN
~Ru
ATOMS @ N b A T O M S
°,~5.0 = -o
,.
Nb-Ru Alloys IO,A,rn present investigation 5.6 Lo,A,m data of Raub & Fritzsche5
-_-_~ \
5.4
~ 4.6 5.2 ~ t3_
5.o ~
(a)
(~)
f.c.t.
Fig. 10. Variation of room temperature lattice parameters of near-equiatomic N b - R u alloys with composition.
CsCI
(c)
f.c.o.
Fig. 9. (a) Untransformed high temperature parent CsC1 type structure in which a f.c.t, cell is delineated with a = x/2a0, c/a = l/x/2, (b) face-centered tetragonal cell obtained on account of transformation distortions; the (equally good) body-centred tetragonal cell is also shown, (c) face-centered orthorhombic cell produced because of a distortion which is anisotropic in the plane perpendicular to the 'c' axis.
this curve shows a direct transformation from fl to fl". The observations of Raub and Fritzsche s corroborate the proposed two-step transformation process. Their data are plotted together with the results of this investigation in Fig. 10 after converting their b.c.t, cells to f.c.t, cells by multiplying their a values by \/2. It can be seen from this Figure that the f.c.o. cell differs very little form the f.c.t, cell and it could be derived from the cubic phase by the suggested two-step process with an intermediate f.c.t, phase. Furthermore, even room temperature c/a and c/b ratios are not very different from 1/x/2=0.707, consistent with the model proposed; c/a and c/b ratios may be very close to 0.707 at transformation temperatures since absence of hysteresis indicates very small strains.
ACKNOWLEDGEMENTS
fl" transformation occurs before completion of fl to fl' transformation; this accounts for the change in slopes of the resistance and magnetic susceptibility versus temperature curves during transformation. This model is also consistent with the hot stage metallographic observations on alloy 2. The appearance of the fine markings (Figs. 5a) arranged in chevron-like bands on the surface polished at room temperature and heated to 750° C is consistent with a reverse reaction if' to fl' while tilting of the broad bands above 800 ° C (Fig. 5b) fits with the fl' to fl reaction. In case of alloy 1 the fl to fl' reaction is complete above room temperature; there may indeed be a ff to fl" transformation below room temperature. In case of alloy 3 it cannot be stated at this time whether a fl to fl' transformation precedes the transformation indicated by Fig. 3, which would then be interpreted as fl' to fl" on cooling, or whether
The authors wish to thank Mr. R. Hoffman and Mr. J. Peters for their aid in crystal preparation and susceptibility measurements respectively.
REFERENCES 1 D. S. LIEBERMAN, The mechanism of phase transformations in crystalline solids, Institute of Metals Monograph No. 33, 167, London, 1969. 2 D. S. LIEBERMAN, Am. Soc. Metals Seminar on Phase Transformations, Detroit, 12-13 October, 1968, in press. 3 P. GREENFIELD AND ]~. A. BECK, Trans. AIME, 206 (1956) 265. 4 A. E. DWIGHT, Trans. AIME, 215 (1959) 283. 5 E. RAUB A~qDW. FRITZSCHE, Z. Metallk., 54 (1963) 317. 6 J. A. GARDNER AND C. P. FLV~qIq, Phil. Mag., 15 (1967) 1233. 7 S.G. FISHMAN, R. KARZ AND D. S. LmBERMAN,to be published. 8 M. A. SCI-IMERLTNG,B. K. DAS AND D. S. LmBERMAN, Met. Trans., in press.
Mater. Sci. Eno., 6 (1970} 248-254